How can I use 1-channel images as input to a CNN?
The libraries should allow you to specify the number of input channels of the convolutional layer, so no one should prevent you from passing 1-channel images as input to a CNN. For example, in PyTorch, you can specify the number of input channels of the Conv2d object. 

If your library does not provide such feature, you could convert your 1-channel images to 3-channel images, where e.g. all 3 channels of each image are equal to the only channel of the corresponding original 1-channel image.